1. Field of the Invention
The present invention relates generally to an exhaust port for a cylinder block of an internal combustion engine. More particularly, the present invention is concerned with a seating mechanism between the exhaust port of the cylinder head of the engine block and a valve.
2. Description of the Related Art
An example of a prior art cylinder head of the engine block apparatus is shown in FIGS. 6 and 7. In FIG. 6, a cylinder head of the engine block 50 is of an internal combustion engine (not shown). A camshaft 51 is rotatably mounted in the cylinder head 50. A valve mechanism 52 is operatively connected to the camshaft 51. A valve shaft 53 is operated in accordance with the operation of the valve mechanism 52. The valve mechanism 52 includes a valve rocker arm 54, a valve stem 55, a retainer 56 and a valve spring 57. The valve shaft 53 is arranged between the valve mechanism 52 and the cylinder head 50. An exhaust conduit 58 is formed in the cylinder head 50. A valve seat 59 is arranged at the opening portion of the exhaust conduit 58. The valve seat 59 is formed of heat-resisting steel.
FIG. 7 shows a partial cross sectional view of a valve seat portion VS of FIG. 6. In FIG. 7, the valve seat 59 is snugly fitted in the cylinder head 50 by a pressing operation.
The valve camshaft 51 rotates in accordance with the operation of the internal combustion engine (not shown). Since the camshaft 51 is provided with sections formed into eccentric, cam-type shape, the valve rocker arm 54 converts a rotational motion into a reciprocating motion about the rocker arm shaft 54a. In accordance with the above mentioned operation, the valve stem 55 pushes down the valve shaft 53. When the valve shaft 53 is pushed down, a cylinder (not shown) and the exhaust conduit 58 communicate each other. On the other hand, when the valve shaft 53 is not pushed down the valve shaft 53 is seated to the valve seat 59.
The valve seat 59 is made of heat-resisting steel and the cylinder head 50 is made of cast aluminum. Based on the difference of the coefficient of heat expansion between the material of the valve seat 59 and the cylinder head 50, a gap may be generated between the cylinder head 50 and the valve seat 59. When the gap occurs, the valve seat 59 may separate from the cylinder head.